Inverters including H-bridge inverters are normally used in applications which require the inversion of direct current to alternating current. Such applications exist when direct current (DC) is readily available to a system but some devices in the system or the system itself operate only on alternating current (AC). Other applications exist where inverters are used to invert a DC voltage to an ac voltage which is then applied to a transformer which steps-up or steps-down the AC voltage for later use. Inverters may be used in modern aircraft to generate alternating current to provide power to radar systems, radio transmitters/receivers and the like, and a low voltage DC is used to charge the battery supply and to power other devices requiring low voltage DC.
In modern aircraft various types of systems can be used to generate power. One example of a conventional power generating system is shown in FIG. 1. In the conventional power generating system 10 of FIG. 1 an integrated drive generator (IDG) 12 is coupled to the drive shaft of the engine of an aircraft. The IDG includes a constant speed transmission which converts varying drive shaft speed into a constant shaft speed. The constant speed transmission is commonly known and can be of the type manufactured by the assignee of the present application.
The IDG also includes an alternator which converts the constant shaft speed produced by the constant speed transmission into 3 phase 400 Hz AC power which is applied to a 3 phase fullwave rectifier 14. The rectifier 14 rectifies the 3 phase, 400 Hz, AC power into a DC voltage which is applied to an H-bridge inverter 16.
The H-bridge inverter 16 inverts the DC voltage from the rectifier 14 into an alternating current which is applied to a transformer 18. The transformer 18 steps-down the AC voltage for rectification by a rectifier 20 which produces 28 volts DC for use by other devices of the aircraft and to charge the airframe battery supply 22.
Conventional H-bridge inverters of the type used in the conventional power generator of FIG. 1 are constructed in the form of the letter "H", as shown in FIG. 2. The conventional H-bridge inverter shown in FIG. 2 includes switches 100, 102, 104 and 106 at each of the legs of the letter "H" and a load 108 connected at the bridge of the letter "H". A DC power source 110 is applied across the legs of the H-bridge inverter such that when the switches are alternately switched, the DC voltage from the DC power source is alternately connected in opposite directions to the load 108. The load is the primary winding of the transformer 18 of FIG. 1. A sensing circuit 112 is provided in the H-bridge inverter to sense the closure of the switches. Sensing of the closure of the switches is performed so that the times at which the switches are closed can be controlled in order to optimize the quality of the AC voltage produced by the H-bridge inverter. The sensing circuit is of the magnetic sensing type which includes a transformer having a first winding 114 connected to a first pair of switches, a second winding 116 connected to a second pair of switches and a third winding 118 connected to a current sensing device 120.
In order to protect the switches of the H-bridge inverter from high currents and voltages during the switching operations, snubber circuits 122, 124, 126 and 128 are respectively connected to the switches.
A snubber circuit, as known, is a suppression network which includes a series connected capacitor and diode. The snubber circuit is connected in parallel to a switch. As indicated above, the snubber circuit protects the switch from high currents and voltages being applied to the switch during switching operations.
In addition, freewheeling diodes 130, 132, 134 and 136 are also connected in parallel with the switches to permit the flow of lagging currents across the switches when the switches are opened.
The conventional H-bridge inverter shown in FIG. 2 operates as follows. At one instant in time a first pair of switches 100 and 106 are closed to complete a circuit through the load 108 and the DC power source 110 thereby providing current through the load 108 in one direction. At another instant in time a second pair of switches 102 and 104 are closed to provide current through the load 108 in the opposite direction. The switches open and close in response to signals from a switch control circuit (not shown) which may operate according to a pulse width modulation technique. By carefully controlling the closures of the pairs of switches an AC waveform can be produced under control of the switch control circuit according to a pulsewidth modulation technique through the load 108.
The sensing circuit 112 senses the times of closure of the switches. Current flowing in the first winding 114 of the sensing circuit 112 when the first pair of switches 100 and 106 are closed induces a current on the third winding 118 which is sensed by the current sensing device 120. Current flowing in the second winding 116 of the sensing circuit 112 when the second pair of switches 102 and 104 are closed induces a current on the third winding 118 which is sensed by the current sensing device 120. A signal indicating the switch closure times is provided by the sensing circuit 112 to the switch control circuit (not shown). The switch control circuit makes use of the signals output by the sensing circuit 112 to determine the times at which the switches are to be closed in order to optimize the operation of the H-bridge inverter.
Thus, as indicated above, the operation of the conventional H-bridge inverter can be optimized by carefully controlling the switch closure times of the switches.
In the conventional H-bridge inverter described above, one major disadvantage occurs. Snubber currents flowing in the snubber circuits 126 and 128 in the lower two legs of the H-bridge inverter parallely connected to switches 104 and 106 also flow into the sensing circuit 112 through lines 138 and 139. These snubber currents effect the operation of the sensing circuit 112 such that the times of switch closure of the pairs of switches is inaccurately sensed. Essentially the flow of snubber currents cause noise in the form of ringing to exist in the sensing circuit 112. The noise is a consequence of ringing in the snubber circuits 126 and 128. For example, when switch 106 opens energy stored in inductor L4 flows to capacitor C4 charging it with the polarity as indicated. Current then flows from the positive side of the capacitor through the power supply 110 and through the winding N1 to the negative side of the capacitor. The current through the winding N1 can interfere with the operation of the sense circuit to properly sense the main current flowing through the legs of the inverter. Ringing of a similar nature is also encountered with the opening of switch 104.
Although various circuits have been proposed for suppressing snubber currents, none have specifically addressed the problem of suppressing noise in the sensing circuit of an H-bridge inverter caused by the flow of snubber currents into the sensing circuit from snubber circuits connected to the switches of an H-bridge inverter.
For example, U.S. Pat. No. 4,542,440 discloses apparatus for switch current sensing in a push-pull converter having circuitry for suppressing snubber currents. As can be seen in the circuitry disclosed by U.S. Pat. No. 4,542,440, the snubber current suppression circuit disclosed therein does not address the problem of noise in the sensing circuit of an H-bridge inverter caused by snubber currents from the snubber circuits.